The present invention relates to a process for the preparation of oxide crystals by means of solution growth. More particularly, the invention relates to a process for preparing, at low temperature, superconductive crystals and analogues having the same crystal structure as do the first-mentioned crystals. Further, the present invention relates to a process for the preparation of oxide crystals, in which superconductive crystals and analogues having the same crystal structure can be produced on a metallic silver substrate.
In preparing high-temperature superconductive oxide crystals and their analogous crystals through solution growth, it is known that stable, large crystals of high quality can be obtained with use of, as a solvent, an oxide melt (a self-flux) of those elements which constitute an oxide crystal to be desired or a melt of mixture consisting of an oxide of those elements which constitute an oxide crystal to be desired and an halide of those elements which constitute an oxide crystal to be desired.
As for the solution growth of crystals, the temperature at which a crystal proceeds to grow is always higher than the temperature at which a solution melts. When a solvent and a solute are held in eutectic relation to each other (in general, a solvent for use in solution growth forms together with a solute an eutectic system), the lower the melt temperature of the solvent is, the melt temperature of the resulting solution becomes lower. In the case of conventional processes for preparing high-temperature superconductive oxide crystals by use of an oxide melt as a solvent, and also for preparing oxide crystals analogous to the first-mentioned crystals by means of solution growth, the melt temperature of the solvent is substantially equal to the eutectic temperature of the mating solute. This is taken to mean that the melt temperature of the conventional oxide solvent is the lowest possible or otherwise critical temperature in a known mode of solution growth using a conventional oxide melt.
In the formation of a YBa.sub.2 Cu.sub.3 O.sub.6+d crystal that is one typical example of high-temperature superconductive oxide crystals, the temperature of crystal growth reaches 970.degree. C. even in its low limit when a 3BaO-5CuO oxide is used as a solvent. Besides, the temperature of crystal growth reaches 920.degree. C. in its low limit when a 3Ba(O+F.sub.2)-5CuO, which is a mixture of an oxide and a halide, is used as a solvent.
Many attempts have been made by the use of certain different solvents, say an oxide melt containing anionic elements other than the elements making up a desired oxide crystal, and halide melt. Most instances, however, fail to achieve adequate crystal growth or provide a crystal as desired, or to produce a crystal involving impure anionic elements. Further, there is a problem that a metallic silver substrate dissolves in a solvent containing an oxide and a halide when the substrate is immersed in the solvent at 925.degree. C. so as to grow high-temperature superconductive oxide crystals on the metallic silver substrate.